Technical Field
The present invention relates to a heat-sensitive transfer recording medium used for a heat-sensitive transfer type printer.
Background Art
Heat-sensitive transfer recording media, which are generally used in many cases in the form of ink ribbons in heat-transfer type printers, are also called thermal ribbons. Such a heat-sensitive transfer recording medium has a structure that includes a base having one surface provided with a heat-sensitive transfer layer and the other surface provided with a heat-resistant lubricating layer (back coat layer). The heat-sensitive transfer layer is a layer of an ink, and the ink of the layer is transferred to an object by sublimation (sublimation transfer method) or melting (melt transfer method) by means of heat generated at a thermal head of a printer.
Of these methods, the sublimation transfer method enables relatively easy full-color formation of various images in combination with a sophisticated printer and thus has been widely used such as for self-prints of digital cameras, cards such as for identification, or output materials for amusement. As the usage of the heat-sensitive transfer recording media is diversified, there arises an increasing need for the media to reduce size, increase speed, reduce cost or enhance durability of the obtained printed materials. For this reason, predominantly prevailing heat-sensitive transfer recording media of recent years include a plurality of heat-sensitive transfer layers which are provided on one surface of a base sheet so as not to be overlaid such as on a protective layer that imparts durability to the photo prints.
Under such circumstances, as printing speed of printers is increasing in association with the diversified and predominantly prevailing usage of heat-sensitive transfer recording media, there arises a problem that the heat-sensitive transfer recording media of the conventional art cannot achieve a sufficient print density. In order to enhance the transfer sensitivity in printing, an attempt has been made to reduce the thickness of such a heat-sensitive transfer recording medium. However, this leads to a problem of causing wrinkles or sometimes a problem of being torn due to the heat, pressure or the like in manufacturing the heat-sensitive transfer recording media or in performing printing using the heat-sensitive transfer recording medium.
Further, in another attempt that has been made, the ratio of dye/binder is increased in the dye layer of a heat-sensitive transfer recording medium to enhance the print density and the transfer sensitivity in printing. However, the increase of dye raises not only a problem of increasing cost, but also a problem of partial transition (offset) of the dye into the heat-resistant lubricating layer of the heat-sensitive transfer recording medium in a state of being taken up in the course of the manufacture. When the heat-sensitive transfer recording medium is rolled again, the dye that has transitioned into the heat-resistant lubricating layer again transitions into a dye layer of a different color or into a protective layer (re-offset). If the smudged layers are heat-transferred to an object to be transferred, the resultant hue may be different from a specified color, or may cause so-called scumming.
Further, in still another attempt that has been made, energy in forming an image is increased on a printer side, not on a heat-sensitive transfer recording medium side. However, in this case, power consumption is increased. In addition, the load imposed on a thermal head of the printer is increased and thus the life of the thermal head is shortened. Further, increase of the load tends to cause uneven thermal conduction of the thermal head and uneven color development in printing, or transfer failure of the heat transferable protective layer. In addition to that, increase of the load tends to cause so-called abnormal transfer that is a fusion between the dye layer and an object to be transferred. In order to prevent the occurrence of the abnormal transfer, the adhesiveness between the base and the dye layer is required to be enhanced. For the purpose of enhancing the adhesiveness between the base and the dye layer, some measures have been taken, such as using a base given an easy-adhesion treatment or providing an adhesive layer (underlying layer) on the base.
The easy-adhesion treatment includes, for example, corona treatment, flame treatment, ozone treatment, ultraviolet treatment, radiation treatment, rough surface treatment, plasma treatment or primer treatment. However, although use of a base given the easy-adhesion treatment can ensure adhesiveness, use of such a base raises a problem of incurring high cost in obtaining the base and of not ensuring sufficient print density.
In order to solve such a problem, for example, Patent Literature 1 or 2 proposes to provide a heat transfer sheet between a base and a dye layer, the heat transfer sheet having an adhesive layer (underlying layer) that contains a polyvinylpyrrolidone resin and a modified polyvinylpyrrolidone resin.
Further, in order to solve the insufficient transfer sensitivity, Patent Literature 3 proposes a heat transfer sheet having an underlying layer which is comprised of polyvinylpyrrolidone/polyvinyl alcohol and colloidal inorganic pigment fine particles.